Rubber vulcanization



Patented July 10, 1951 RUBBER VULCANIZATION George E. P. Smith, Jr.,Akron, Ohio, assignor to The Firestone Tire & Rubber- Company, Akron,Ohio, a corporation of Ohio No Drawing. Application June 21, 1949,Serial No. 100,548

3 Claims. (Cl. 260788) This invention relates to the vulcanization of arubber in the presence of a new class of chemical compounds which arevaluable accelerators for the vulcanization of natural or syntheticvulcanizable rubbers. This application i a continuationin-part of mycopending application Serial No. 543,054, filed June 30, 1944, nowabandoned.

The objects of the invention are to provide a method of vulcanizin arubber whereby scorching or pre-vulcanization during processing of therubber composition is prevented, but extremely rapid vulcanization isobtained at customary vulcanizing temperatures; to provide rubbercompositions possessing the ability to vulcanize extremely rapidly atordinary vulcanizing temperatures, but free from any tendency to set-upor pre-vulcanize at rubber processing temperatures; to provide avulcanized rubber of improved quality, possessing the characteristics ofhigh modu lus and tensile, low hysteresis and a high inherent resistanceto deterioration by flexin or aging.

The above and further objects will be manifest in the description of theinvention which follows.

The objects of the invention are realized through the discovery and useof a new class of chemical compounds which are derivatives ofsulfenamide (HSNI-Iz). The compounds may be conveniently prepared byoxidizing a mixture of a Z-mercapto-thiazole and a primary aminecontaining an alkenyl radical. The oxidation may be effected b treatinga mixture of the amine and the mercaptothiazole in an alkaline aqueousmedium with an oxidizing agent, such as chlorine, bromine, iodine,hypochlorous acid, hypobromous acid, hypoiodous acid, or alkali metal oralkaline earth metal salts of said acids. The new class of productsproduced possesses the following formula:

wherein R1 and R2 are hydrogen or radicals derived from hydrocarbons ortogether represent a cyclic radical, and R3 is an alkenyl orcycloalkenyl radical.

Examples of amines which may be used in forming the accelerators includethe following:

Gamma methyl crotonyl amine Isobutenyl amine 2,3,4,5-tetrahydrobenzylamine Alpha,beta-dimethylallyl amine Beta,gamma-cyclohexenyl amineGamma,delta-butenyl amine Gamma,delta-cyclohexenyl amine Isopropenylamine Alpha,beta-cyclohexenyl amine Isocrotonyl amine TheZ-mercaptothiazole employed may be an aromatic thiazole, wherein R1 andR2 of the above formula represents an ortho-arylene radical. Examples ofortho-arylene radicals include benzo, l-phenylbenzo, G-phenylbenzo,naphtho, and homologs or the various ring substitution products of theseradicals; as typical ring substituents there may be mentioned nitro,halogen, hydroxy and alkoxy groups. R1 and R2 may together form anon-aromatic divalent radical, such as the various hydrogenated orpartially hydrogenated benzo radicals; examples of mercaptothiazoles ofthis type are 2-mercaptotetrahydro-benzothiazole and ring substitutionproducts thereof. The mercaptothiazole may, however, be a member of thesocalled non-aromatic thiazoles, some examples being the following:

Z-mercaptothiazole 2-mercapto-4-methyl-thiazole2-mercapto-4,5-dimethyl-thiazole 2-mercapto-4-ethyl-thiazole2-mercapto-4*phenyl-thiazole 2-mercapto-4-phenyl-5-methyl-thiazole2-mercapto-4-propyl-thiazole The following examplesare given in furtherillustration of the invention.

Example 1 To 10.75 grams of commercial 2-mercaptobenzothiazole therewere added 5.5 grams of solid sodium hydroxide dissolved in about ml. ofwater. After reaction was complete the solution was filtered. Theresulting clear solution of the mercaptothiazole was mixed with 42.5grams of primary isobutenyl amine, and the mixture was diluted withabout 300 ml. of water. The'alkaline mixture was then treated with asolution of 15.2 grams of iodine and 20 grams of potassium iodide inwater, by adding the oxidizing iodine solution dropwise to the agitatedalkaline mixture. A white solid precipitated durin the addition of theiodine. After the addition of all the iodine, the precipitat wascollected, washed with water and dried, resulting in 9.5 grams ofproduct. The latter was purified by recrystallization from petroleumether. The purified substance melted at 64-66 and isN-isobutenyl-Z-benzothiazole sulfenamide, having the formula Thepurified product analyzed 11.76% nitrogen and 26.97% sulfur; the valuescalculated from the above formula are 11.86% nitrogen and 27.13% sulfur.

The sulfenamide, so prepared, was tested as an accelerator by comparingit with the standard accelerators, mercaptobenzothiazole and2,2-dithiobis-benzothiazole, the latter being a delayedactionaccelerator, in the followin rubber formula Ingredients: Parts by weightRubber (smoked sheets) 100.00 Sulfur 3.00 Zinc oxide 5.00 Stearic acid1.10 Accelerator 0.75

Samples of the three rubber compositions, so formulated, were heated for60 minutes at 240 F. to determine whether or not they possessed anytendency to pre-vulcanize or set-up during processing steps prior to theusual vulcanizing step, and the following physical testing data wereobtained:

Modulus of Elasticity Accelerator in lbs/in. in lbs 3 at elongaat tionof 600% ltlercaptobenzothiazole 750 2, 800 2,2-Dithiobis-benzothazole Nocure No cme N-Isobutcnyl-Q-benzothiazole sullenamida N euro No cureOther samples of the aforesaid rubber compositions were heated for 40minutes at 280 F. to determine the relative accelerating properties ofthe respective accelerators and the following data were obtained:

Modulus of Elasticity gij Accelerator in lbsJin. 1h

at elongoi tion of 600% a Mcrcaptobenzothiazole 650 2. 7002,2-Dithiobis-benzothiazole i. 750 3, 150 N-Isobutenyl-Z-benzothiazolesull'enamide. 2, 000 3, 675

Example 2 To 57.09 grams (1.00 mole) of allylamine there was addedslowly and with stirring, at a temperature of 10 C., 210 ml. of 2.91molar sodium hypochlorite solution (0.625 mole). As soon as the additionof the hypochlorite solution was complete, :350 ml. of an aqueoussolution of 0.5 mole of the sodium salt of Z-mercaptobenzothiazole wasadded to the cold reaction mixture. The solid reaction productprecipitated and was separated by filtration. After severalrecrystallizations from gasoline the product melted sharply at til-62 C.The product, N-allyl-2-benzothiazole sulfenamide, has the followingstructure:

CS-l\CH CH=CH In accordance with either the method of Example 1 or themethod of Example 2, crotonyl amine may be reacted withZ-mercaptobenzothiazole to produce N-crotonyl-2-benzothiazolesulfenamide of the following formula:

These two additional sulfenamides are excellent delayed-actionaccelerators, having similar properties to N-isobutenyl-2-benzothiazolesulfenamide. Other sulfenamides of the invention may be prepared inanalogous manners by oxidizing the mercaptothiazole with the appropriateprimary amine, or by reacting, the disulfide of the mercaptothiazolewith the amine. Thus, for example, the following active delayed-actionac celerators may be so produced:

N-tetrahydrobenzyl-Z-thiazole sulfenamide Nisobutenyl-2-tetrahydrobenzothiazole sulfen amide The invention is notlimited to derivatives prepared in accordance with any particularmethod, but includes vulcanization in the presence of the preferredclass of chemical compounds, however they may have been manufactured.

The new accelerators are equally efiective in rubber tread compounds,latex compounds or other conventional vulcanizable compounds of rubberor synthetic rubber. The various vulcanizable synthetic rubbers areherein considered equivalent to rubber, examples being the rubberypolymers essentially derived from 1,3-butadiene, such as copolymers ofbutadiene and acrylonitrile and copolymers of butadiene and styrene.Other examples are polybutadiene rubber, polyisoprene synthetic rubbersand rubberycopolymers of a butylene and a diolefin. Although sulfur hashereinabove been men tioned as the vulcanizing agent preferred inpracticing the invention, other conventional sulfur-bearing vulcanizingagents susceptible to acceleration are contemplated, such as theso-called sulfur donor" type of vulcanizing agents, of whichtetramethylthiuram disulfide, polysulfidts of phenols and cresols, anddiethyl Xanthogen disulfide are Well-known examples. The accelerators ofthe invention accelerate the vulcanization of a rubber by means of suchsulfur donors even though no added free sulfur is present in the rubbercomposition. Whether the sulfur donors break down to give free sulfurduring the vulcanizing step is unknown, but they do contain sulfur andthe invention is considered to cover their use as well as the use offree sulfur.

The preferred compounds are seen to be very rapid accelerators atconventional vulcanizing temperatures, imparting unusually high modulus:and tensile characteristics to the vulcanized rubber, but they are ofthe delayed action type, in that vulcanizable compositions includingthem do not set-up or pre-vulcanize under conventional rubber processingconditions. The vulcanized products are also highly efficient, havinglow hysteresis characteristics, and are unusually resistant todeterioration on flexing or aging.

Conventional accelerator activators may be employed to an advantage withthe new class of accelerators when extremely rapid vulcanization isdesired, especially at lower temperatures. Also, the new acceleratorsmay be used in combination with other accelerators, such as a guanidine(e. g., diphenyl guanidine) or a conventional mercaptothiazolederivative (e. g., mercaptobenzothiazole or2,2'-dithiobis-benzothiazole), in order to produce rubber compositionshaving especially rapid or specific vulcanizing properties.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,268,467 Ashworth Dec. 30, 19412,339,552 Carr Jan. 18, 1944 2,417,989 Moore et a1. Mar. 25, 19472,445,722 Carr et a1. July 20, 1948 2,460,393 Paul Feb. 1, 1949

1. THE METHOD OF VULCANIZING RUBBER WHICH INCLUDES HEATING A RUBBER ANDSURFUR IN THE PRESENCE OF N-ISOBUTENYL-2-BENXOTHIAXOLE SULFENAMIDE.